Contrary to popular belief, Mac OS X does have a 'blue screen of death', only it's grey instead of blue, and it's called the kernel panic screen, and it sure does scare the pajeezers out of you if you just bought a Mac (sadly, I can know), and it has had this screen since 10.2; before that, the error information was spilled as raw data straight on the screen. Amit Singh writes about the history and workings of this kernel panic screen, and provides methods of modifying it, or turning it into the 'raw data baby' version. And yes, you can even make it look exactly like the real BSOD. Die-hard Windowfying OS X, this.

"In Linux when a desktop machine comes to a dead stop, it's usually X going mental. Sometimes control+alt.+backspace kills/restarts X, however, if X is seriously lost, that key combo won't work since X drives the keyboard as well."

At least in those situations, you can log in via ssh from another machine and kill the offending processes. As long as nothing too awful has happened of course!